Slightly niche but - I pre-booked parking at Heathrow Airport in Feb with a HUKD code. Thought 18 days for £94.00 was ok (didn't forget Quidco). Just re-checked and the new price today was £72.00. Simply went into NCP account amended booking and got a £22.00 credit. Happy days.
Be aware though - your previous booking may be flexible and have a 100% refund policy where the new policy will be non-refundable.https://www.ncp.co.uk/